Many industrial applications require large scale generation of heat from burners for process heaters, boilers, or other fired heating systems. If the burner fuel is thoroughly mixed with air and combustion occurs under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, such as in a high temperature environment, nitrogen present in the combustion air reacts with oxygen to produce nitrogen oxides (NOx). It is well known that, other conditions being equal, NOx production increases as the temperature of the combustion process increases. NOx emissions are generally considered to contribute to ozone depletion and other environmental concerns.
Burners designed for combusting fuel with air in a manner resulting in reduced NOx emissions are commonly referred to as “low NOx” burners. In some low NOx burners now used in the art, flue gases produced in the combustion process are recirculated through the burner apparatus to dilute the fuel/air mixture in the combustion zone and thereby lower the burner flame temperature. However, when the flue gas is recirculated from the interior of the fired heater without significant cooling, the temperature of the recirculated flue gas is still extremely high when it enters the combustion zone. The high temperature of the recirculated flue gas itself thus counteracts some of the beneficial dilution effect of the recirculated flue gas and limits the amount of NOx reduction achieved.
A need presently exists for an efficient, improved low NOx burner apparatus and method utilizing recirculated flue gas wherein the temperature of the flue gas is significantly reduced prior to being delivered into the burner flame, but without losing any heat energy from the combustion system. A need particularly exists for an improved low NOx burner apparatus and method of this type which is not complex in design and which provides stable performance over a broad range of operating conditions. In addition, a need exists for a burner apparatus and method of this type which can be operated in a manner effective for significantly increasing the amount of recirculated flue gas used in the burner.
U.S. Pat. No. 5,458,481 discloses a staged fuel burner having a primary combustion zone within the central passageway of the burner block (i.e., the burner wall) and a secondary fuel combustion stage provided at the outer end of the block. The burner block includes a plurality of vertical flue gas recirculation passageways around the central passageway. Each of the vertical recirculation passageways has a lateral injection passageway at the bottom thereof which extends into the central passageway. Fuel jets are positioned in the lower ends of the vertical recirculation passageways for injecting primary fuel gas into the lateral injection passageways. The injection of the primary fuel gas draws flue gas through the vertical recirculation passageways and then carries the flue gas through the lateral injection passageways.
In the burner of U.S. Pat. No. 5,458,481, the injected primary fuel gas undergoes localized combustion in the presence of the inert recirculated flue gas in a primary combustion region within the central passageway of the burner wall at the discharge outlets of the lateral injection passageways. Directing structures are positioned within the burner adjacent to the discharge openings of the lateral injection passageways for separating the local combustion of the primary fuel gas from the main body of air flowing through the central passageway. Each of the directing structures has a perforated bottom plate which allows a sufficient amount of air to pass therethrough for supporting the localized combustion of the primary fuel gas. A plurality of secondary fuel jets are provided at the forward end of the burner block for ejecting secondary fuel gas laterally into the secondary combustion zone at the end of the burner block.
U.S. Pat. No. 6,499,990 discloses a particularly effective low NOx burner apparatus and method wherein the burner preferably has only a single combustion zone. The combustion zone is located at the forward end of the burner wall and receives combustion air via an interior passageway having an outlet at the forward end. Fuel gas is delivered to the combustion zone by a plurality of ejectors which are positioned outside and rearwardly of the forward end of the burner wall. The fuel gas is ejected outside of the burner wall in a plurality of free jet flow paths such that flue gas is entrained in the fuel gas as it travels toward the combustion zone. The burner can also comprise one or more exterior impact structures positioned to assist in further mixing the flue gas with the fuel. The impact structure(s) can comprise(s) one or more ledges formed on the exterior of the burner wall. Alternatively, the exterior of the burner can be sloped (e.g., frustoconical) or straight (e.g., cylindrical). The entire disclosure of U.S. Pat. No. 6,499,990 is incorporated herein by reference.
The manner of ejection and delivery of fuel gas outside of the burner wall as employed in U.S. Pat. No. 6,499,990 is effective for entraining a sufficient amount of inert flue gas in the fuel gas stream to significantly reduce NOx emissions. Moreover, because of its single-stage design and operation, the burner of U.S. Pat. No. 6,499,990 is less complex than other low NOx burners previously known in the art and is also safer, more stable, and simpler to operate, control, and maintain. In addition, the burner of U.S. Pat. No. 6,499,990 provides a much broader stable operating range (turn down ratio) than is available with staged fuel or staged air burners and can be more readily and conveniently sized and adapted for use in different fired heating systems.
U.S. Pat. No. 6,695,609 discloses a low NOx, staged fuel burner wherein the exterior of the burner wall is divided by radial baffles into a series of alternating sections which have different heights and which slant inwardly toward the outer end of the burner wall at different angles. Some of the wall sections have a lateral opening formed through the base thereof through which primary fuel gas is injected. The primary fuel gas entrains an amount of flue gas from the heating system and carries the flue gas into the burner. The primary fuel gas/flue gas mixture is combusted within the burner wall beginning in the vicinity of the lateral openings. A radial ledge positioned within the burner just below the lateral openings functions as a flame stabilizing surface for the primary combustion zone. Secondary fuel gas is discharged outside of the burner wall such that the secondary fuel gas entrains additional flue gas as it travels toward a secondary combustion zone located at the forward end of the burner.